White spot disease (WSD), the synonymous of penaeid acute viremia (PAV), has become one of the most serious problems of the shrimp culture business in the world. One important infection route of the causative virus (WSSV: white spot syndrome virus = PRDV: penaeid rod-shaped DNA virus) is a vertical transmission from spawner to larvae via eggs in the seed production process of kuruma shrimp (Marsupenaeus japonicus). To prevent the vertical transmission, disinfection methods of fertilized eggs and high-sensitive detection methods for the causative virus by molecular technologies have been developed. Until recently, the seed production has been depending on the mated and captured female broodstock. However, as WSSV is spontaneously affecting the broodstock in nature at some level, the vertical infection occurs due to the virus replication in the virus-carrier female broodstock caused by spawning stress. Therefore, specific pathogen free (SPF) kuruma shrimp stock is highly important for prevention of WSD and for sustainable shrimp culture in Japan.

Vibriosis is the general term for diseases caused by bacteria belonging to the genus Vibrio. “Red-pest” of eels caused by Vibrio anguillarum is the first description of fish vibriosis, and 14 Vibrio species have been known as the etiological agent of aquatic organisms. Vibrios are basically halophilic and distributed widely in ocean, and the disease occurs mainly in marine cultured fish and shellfish worldwide, except for some infections in freshwater cultured salmonids and ayu. The disease also has posed severe problems in hatchery-produced larvae and juveniles. In this review, we briefly outline vibriosis particularly important for Japanese aquaculture.

Aeromonas salmonicida subsp. salmonicida was originally described agent of furunculosis in salmonid fish at the end of 19th century, while atypical A. salmonicida has been isolated from a wide range of cultivated and wild fish species, non-salmonids as well as salmonids, inhabiting fresh water, brackish water and marine environments. Although A. salmonicida subsp. salmonicida is homogeneous, atypical strains are more heterogeneous with respect to biochemical characteristics and growth conditions. Based on the nucleotide sequences of 16S rDNA, Japanese isolates of atypical A. salmoicida from different fish species can be divided into four groups. This review describes the outlines and the topics of the diseases.

Blood flukes infecting tiger puffer Takifugu rubripes were re-examined taxonomically. Psettarium sp. TPC Ogawa et al., 2007, Psettarium sp. TPJ Ogawa et al., 2007 and P. japonicum sensu Yamaguti, 1938 were analysed, together with the type specimens of P. sinense (Liu, 1997), originally recorded in Takifugu oblongus as Paradeontacylix sinensis Liu, 1997. Psettarium sp. TPC, represented by blood flukes from T. rubripes cultured in Dailian, China and those collected from T. rubripes imported from China and cultured in Kagawa Prefecture, Japan in 2005, were synonymized with P. sinense. Psettarium sp. TPJ, collected in 1993 from T. rubripes caught in Wakasa Bay, Fukui Prefecture and maintained in a cage for several months, was different from the congeners in the combination of having a large body size (larger than 7 mm long), a single testis extending posterior to the ovary, presence of anterior and posterior vasa deferentia, and is described here as P. wakasaense n. sp. A single museum specimen of P. japonicum sensu Yamaguti, 1938 collected from T. rubripes off Shanghai was morphologically distinguishable from most similar P. sinense in the dextral ovary, the female pore opening anterior to the cirrus pouch and absence of the testis on dextral side of the ovary, and is proposed here as P. yamagutii n. sp.

We examined the amphidoromous, land-locked, and released (farmed) ayu in the Kagami River for Flavobacterium psychrophilum infections using TaqMan real-time PCR targeting the PPIC gene. We collected 524 ayu in 2014-2015 at 4 stations in the lower reaches and 5 stations in the upper reaches of Kagami Dam. The prevalence of infection in apparently healthy individuals did not differ between amphidromous and released fish in the lower reaches or between land-locked and released fish in the upper reaches. However, most dead fish were released ayu. The most commonly isolated F. psychrophilum genotype from ayu was A/G-C type. Ayu carrying F. psychrophilum were found in mid-June 2014, before ayu fishing season started, at two stations in the upper reaches of the dam. F. psychrophilum with genotype A/G-T was detected in 2014-2015 at one of these two stations, suggesting that F. psychrophilum reservoirs may be present and part of the overwintering population that contributes to an increase in infection rates in the next year. It is possible that land-locked ayu larvae and juveniles are F. psychrophilum reservoirs, because the PPIC gene (over 105 copies/L) were detected in the river water collected at the station where eggs were laid in the 2015 spawning season.

Morphological and molecular comparisons of Kudoa septempunctata isolates from 28 olive flounder Paralichthys olivaceus (19 from Japan, nine from Korea) were conducted to determine the geographical origin of infected flounders. Morphological examination showed that the frequency of spores with six polar capsules (PCs) was the highest (32%-86%) in all isolates examined, although there was a significant difference in the frequency of spores with seven PCs between Japanese (mean 2%, range 0%-15%) and Korean (mean 44%, range 12%-61%) isolates. The factor analysis showed that neither body weight of the host fish nor sampling time was significantly correlated to the frequency of spores with seven PCs. These results suggest that the frequency of spores with seven PCs can be used as an indicator to differentiate between Japanese and Korean isolates of K. septempunctata. In the molecular analysis, the isolates from the two countries had indistinguishable 18S-28S internal transcribed spacer (ITS) regions. However, comparative analysis of cytochrome b and NADH dehydrogenase subunit 1 region of mitochondrial DNA revealed that intraspecific variations existed within the Japanese and the Korean isolates, as well as in two isolates associated with food poisoning of unknown origin. Furthermore, the PCR-RFLP analysis on the region using the restriction enzymes Hpy188I and Hpy188III indicated that there were four types among the isolates. These morphological and molecular indicators may be useful to discriminate between the two geographical origins of K. septempunctata.